Natural organic waste matters such as garbage, sewage sludge, pulp industry sludge, waste matter from primary industries and farming and fishing villages, and waste matter from food factories, have been subjected to anaerobic decomposition in fermentation tanks containing methane bacteria, and the methane gas generated in the tanks have been recovered. However, such anaerobic decomposition has generally been quite inefficient because of the very long period of time (generally longer than 30 days) required to attain a equilibrium stage in the methane fermentation. This is due to the fact that anaerobic decomposition comprises two stages, namely: (1) acid fermentation, and (2) methane fermentation.